Film-type heating element



July 13, 1954 H. 1. slLvr-:RsHER FILM-TYPE HEATING ELEMENT Filed March l0. 1952 Patented July 13, 1954 UNITED STATES PATENT OFFICE FILM-TYPE HEATING ELEMENT Herman I. Silversher, Los Angeles, Calif., assigner to Electrofilm Corporation, North Hollywood, Calif., a corporation of California Application March 10, 1952, Serial No. 275,780

(C1. IVY-71) 11 Claims. *l

This invention has to do with improvements in heating devices and heating elements of the type employing a uniform and intimate adminture of finely divided electrically conductive particles and relatively high resistance ller particles, held by a binder in current-passing relation. in certain oi' its more specic aspects the invention relates to nlm dimension heating elements of this general composition, and to flexible heating in the form of a temperature resistant base layer, sheet or strip to which is bonded the heating nlm. Although adaptable to different specinc purposes and forms, the invention has an important field of use as a ilexible composite sheet or tape applicable to pipes, vessels and various other objects requiring surface or wall heating, and therefore will be described as embodied in this typical iorm.

One of my general objects is to provide a novel heating element having the combined properties and advantages of (1) complete flexibility for application to various surface shapes, (2) stability in its physical, thermal and electrical properties over long periods of time, and (3) capacity for heating at temperatures Within the range of about 100 F. and upwards.

For better understanding of the particular objects and accomplishments of the invention, reference may be had preliminarily to the general nature and composition of the electrically conductive and resistive particles in the heating element. in its broader aspects the invention cnntemplates the of any suitable inely divided parables in such proximate and current passing relation will produce a predetermined teniperature at given potential. Preferably I employ an intimate a uniform mixture of conductive metallic par s and relatively high resistance filler particles, all oi which icr best results are preferably below lo microns in their greatest dimensions. Maintenance of contact between the particles despite ilexure or thermally induced dimensional changes in the element, and stabilication therefore of the electrical and thermal properties ci the element, may be assured to advantage by using metal particles in flake forni, or mixtures of metal particles some atleast of which have flaky form.

Typical metals include silver ake (e. g. product No. MTD-585 of Metals Disintegrating Company of Elizabeth, New Jersey), nickel ilalse (e. g. Metal Disintegrating Companys product MD-'750) zinc precipitate (e. g. "lt/ierrillite sold by Alloys Division oi Metals Disintegrating Company, of Berkeley, California), and inely divided powdered or flaky stainless steel. Copper, aluminum and various metal alloys may be used, depending upon the particular electrical and thermal properties desired for a particular heating element. As obtainable, these powdered metals may range in size as high as t microns, but the minus 10 micron and preferably minus S micron sizes have been used most successfully for the purposes of the present invention. O1 the various metals silver may be regarded as preferred.

The relatively high resistance powdered filler may include any or mixtures of the following: titanium oxide, cahiuzn carbonate, calcium silicate, talc and silica. Other materials having corresponding resistance pr perties may be used. For certain purposes, less resistive materials such carbon and graphite may be used.

Generally speaking, the proportions of and filler particles may range within rather wide limits, depending upon the resistance and hear ing characteristics desired. For example in a low resistance element designed to operate at about Soc F., at 21,2 watts per square inch, the mixture may contain about parts of metal (e. g. silver) to parts of illler. This ratio may vary typically to around 90 parts oi' metal to 6o parts of ller in a high resistance element designed to operate at corresponding temperature and wattage.

The invention is more particularly concerned with the manner in which the heating element or lm particles are bonded together in stable condition, and with so supporting the element, particularly where the latter is in thin sheet or strip form, as to provide a composite element and support having the degree of free ilexibility adapting it to various surface application us In this aspect the invention contemplates an iinproved heating element having elastomeric properties by using as a particle binder, a high ternperature resistant elastomer'. Also contempl: d in conjunction with an elastomeric heating element, particularly in a thin or lrf-lilte iorzn, is the use of a heat-resistant elastomer body or carrier, bonded to and in effect integrated with the heating element.

Successful results have been achieved using as the elastomeric material, the generally-termed silicone rubbers, the commercial unpolymerized gum or resin grades of which, sold usually in a paste form, be polymerised by heating es permanent elastomers. .lthough for present purposes these silicone rubbers need not be discussed in chemical detail, they may be characterized in accordance with their known structures as being macro-molecular alkyl di-substituted poly siloxanes copolymerized in an elastomer structure. As examples of useable commercial grades or brands of polymerizable pastes, I have used Dow-Corning Silastic-125, believed to be a mixture of the gum or polymerizable silicones together with ller .particles andf'an;zaccelerator, and General Electric CompanysS; E.+'76. gum, a clear polymerizable silicone material to which I lO add the ller particles and accelerator. Theseffl polysiloxane elastomers are to be distinguished. from the non-elastomeric resins, whichA are un`4 suited for the purposes of the:invention.;as.-.the conductive particle binder materiaux..

In compounding the metal and filler Vpartie-levi: mixture with a polymerizable silicone, material in which filler particles have not been incorporated, the weight ratio of the silicone material orfpaste' to the combined metal and'ller particles (which 2u are. relatively proportioned Vas ,previously indicated) may vary from to .50,parts `of thesili-. cone material for 100 parts of the solid particles, theobject in most instances-being to use no. more than enough of the siliconesto give an eiective. 25 elastomer binder for the particles`. As. willfap.-V pear, formationcr" the element. may.resu1t from4 heating of silicone-filler.andmetal particle mixture containing a suitable polymerization ac celerator such as benzoyl peroxide. l

Further details concerning an illustrative embodiment of the invention in.anelongated strip or tape form of heating element, will be further understood -from the' following .detailed descrip-` tion of the accompanying drawing; ,in'which: 35

Fig. l is a View .showing the composite heating tape applied to a pipe Fig. 2 is a plan view of thetape .compositepwith' the layers progressively broken away toillustra-te their relationship; andv 40 Fig.,3 is a cross-sectionon lines-I3 of Fig; .2.

Referring to Figs. 2 and3, the baseer supporting layer le'consists essentiallyof the silicone elastomer. in which is embedded suitable re.- eni'orcement, preferably in they nature of a heatresistant woven fabric: As illustrative', the layer.

l@ may comprise a sheet or tape of glass cloth i I embedded in the elastomer andformed byini-s tially coating both surfacesA of theglass cloth` at uniform thickness'with'thesilicone paste (containing an accelerator) and then heating they base strip to polymerizeand elastomerize the silicone. If for any reasondesredrsuitable Yiller may be incorporatedV in the rubberr Typically)- the thickness of the'base layerv'l may range between 0.005150 au15-inch. 55

Theheating element generally-indicatedat l2 comprises a thin layer-or coatingA I3 which asv-previously indicated, comprises-in itsnalcondition a uniform mixture of the metal andlfiller-.powdersi in the silicone elastomer binderwhich in turn is v bonded to and in effect-integrated withlthe base layer ll. L It will be noted that the Vheating ele-- ment lrn l2-is effectively:insulatedlectrically from anymetalsurface .to whichtthe'..tapeom.-.v

posite may be applied: in that thelielement. strip'. .65.

is narrower than the 1 baserlayer. and.' ordinarily. will be givenequal spacing atwl vfrom#:the/edges of the base.

in manufacture or" ythe composite; thefsiliclona; powdered Ametal and ller, togetherfwithf'henzoyl 4t7() peroxide, may be appliedn-to theibase. Lbyspray.- ing, brushing or knifegcoatng' the paste'.mixture;-v at uniform thickness. and inspaced;relatiorrato..: the edges'of the base-.r3V Followingapplicationfof i:

the heating element nlm, thetape :may;be;heate:15:75.fA

under temperature and time conditions (typically at 500 F. over a period of one hour) sufficient to polymerize the siliccnes to their elastomeric form. The resulting film thus has an essentially elastomer body both in the base layer and heating element lm, giving the composite complete flexibility and ready applicability to surfaces of varous shapes.

As illustrative, the heating tape may be used for heating a pipe l, as illustrated in Fig. 1, wherein the tape is shown to be wrapped spirally aboutthe .pipe and electrically connected at its ends to suitable poles or current conductors i6. The 'latter-*are shown-"to be attached to copper :mesh conductciui.I fplaced about the end convolutions cf-thetape direct contact with the heating film i3. vAt the location of the conductors, the end'convolutions of the tape may be electrically insulated from the pipe by a layer of asbestos cloth or other suitable insulating materialY i8 pi between the .tape the pipe. In order to prevent short circuiting or electrical bleed o from the hlm it, the strip composite may carry a surfacel insulation layer la? suitably bonded to the surfacesbeneath and composed or example of berglass and asbestos sheet, such as.l\`.fovabestos manufactured Irvington Varnish .Company vof El' Monte, California. For clarity in illustrating the base layer and heating lm in Fig. l, showing .of any surface installation i9 has been omitted.

Thefollowing are illustrative examples of the composition. andV resistance characteristics or" heating films which may be laid upon silicone rubberor other base supports, as discussed in thefforegoing.

Example II Moderately low resistance element:

Silastic` (Dow-Corning) 8l gms. Silver- Flake; 69 gms. Xylene l5()` gms. Resistance" approx. 0.249 hm./ sq. in. Thickness @.Oeinch Example III SE '76 (GeneralEle'c. Silicone Elastomer) 15 gms..

`Antirriony. Oxide 20 gms. Silver ]3lal :e Y 35 gms. Xylene gms. BenzoylV Perox -e 1.5 gms. Resistance appro 0.29 hm./sq. in. Thickness 6.045 inch The' 'resistance and' thickness v values given above areiof course thoseo the heat-set or poly7 merized inishedmixture,

I claim? l. A heating :element comprising a uniform mixture of fnnely divided metallic electrically conductive particles and 1eiatively-high electrically-resistive -ile'r particles all bonded together 1n proximateand current-passingrelation by a heat resistant elastomereo eopolymerized poly v siloxanes, anda supporting member bonded to saidzelement' and composed ci a heat resistant l elastomer 'of Ioopclyrnerized, poly siloxanes;

2. A heatingelementrcomprising a. freely flexible body composed of a uniform mixture of finely divided metallic electrically conductive particles and relatively high electrically resistive filler particles all bonded together in proximate and current-passing relation by a heat resistant elastomer of copolymerized poly siloxanes, and a flexible supporting member bonded to said element and composed of a heat resistant elastomer of copolymerized poly siloxanes.

3. The combination including a base layer having an extended fiat surface and composed of a heat resistant elastomer of a copolymerized poly siloxane and a heating element layer bonded to said base layer surface and comprisng a uniform mixture of finely divided metallic electrically conductive particles and relatively high electrically resistive ller particles all bonded together in proximate and current-passing relation by a heat resistant elastomer of copolymerized poly siloxanes.

4. The combination including a thin exible base layer of heat resistant elastomer of copolymerized poly siloxanes, reenforcing woven glass fabric embedded in said elastomer, and a thin flexible heating element coating bonded to said base layer and comprising a uniform mixture of finely divided electrically conductive particles and relatively high electrically resistive filler particles all bonded together in proximate and current passing relation by a heat resistant elastomer.

5. A heating element comprisng a uniform mixture of finely divided metallic electrically conductive particles and relatively high electrically resistive ller particles all bonded together in proximate and current-passing relation by a heat resistant elastomer of copolymerzed poly siloxanes, said conductive and filler particles being less than microns in their maximum dimensions, and a supporting base member bonded to said element and composed of a heat resistant elastomer of copolymerized poly siloxanes.

6. A heating element comprisng a uniform mixture of finely divided metallic electrically conductive particles and relatively high electrically resistive ller particles all bonded together in proximate and current-passing relation by a heat resistant elastomer of copolymerized poly siloxanes, said metallic particles being in flake form and both the conductive and filler particles being less than 10 microns in their maximum dimensions, and a supporting base member bonded to said element and composed of a heat resistant elastomer of copolymerized poly siloxanes.

7. A heating element comprising a uniform mixture of finely divided electrically conductive flake silver particles and relatively high electrically resistive filler particles all bonded together in proximate and current-passing relation by a heat resistant elastomer of copolymerized poly siloxanes, and a supporting base member bonded to said element.

8. The combination including a thin flexible base layer of heat resistant material and a thin flexible heating element coating bonded to said base layer and comprising a uniform mixture of finely divided electrically conductive flake silver particles and relatively high electrically resistive filler particles all bonded together in proximate and current passing relation by a heat resistant elastomer.

9. The combination including a thin flexible base layer of heat resistant copolymerized poly sloxane elastomer, and a flexible heating ele ment coating of about 0.030 to 0.055 inch thickness bonded directly to said base layer and comprising a uniform mixture of finely divided electrically conductive flaky metal particles and relatively high electrically resistant filler particles all bonded together in proximate and current passing relation by copolymerized poly siloxane elastomer so that the composite base layer and coating are freely flexible, said conductive and filler particles being less than 10 microns in their maximum dimensions.

10. The combination including a thin exible base layer of heat resistant copolymerized poly siloxane elastomer, woven glass fabric embedded in said layer, and a lexible heating element coating of about 0.030 to 0.055 inch thickness bonded directly to said base layer and comprising a uniform mixture of finely divided electrically conductive laky metal particles and relatively high electrically resistant filler particles all bonded together in proximate and current passing relation by copolymerized poly siloxane elastomer so that the composite base layer and coating are freely exble, said conductive and filler particles being less than 10 microns in their maximum dimensions.

11. The combination including a thin exible base layer of heat resistant material composed of copolymerized poly siloxanes and a thin exible heating element coating bonded to said base layer and comprising a uniform mixture of finely divided electrically conductive flake silver particles and relatively high electrically resistive graphite particles all bonded together in proximate and current passing relation by a heat resistant elastomer of copolymerized poly siloxanes, said silver and graphite particles being predominately under 10 microns in size.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 940,151 Heath Nov. 16, 1909 2,403,657 Harvey July 9, 1946 2,415,036 Quinn Jan. 28, 1947 2,448,756 Agens Sept. 7, 1948 2,460,795 Warrick Feb. 1, 1949 2,473,183 Watson June 14, 1949 2,526,059 Zabel Oct. 17, 1950 2,559,077 Johnson et al. July 3, 1951 2,609,316 Fichtner Sept. 2, 1952 FOREIGN PATENTS Number Country Date 581,212 Great Britain Oct. 4, 1946 627,241 Great Britain Aug. 3, 1949 

1. A HEATING ELEMENT COMPRISING A UNIFORM MIXTURE OF FINELY DIVIDED METALLIC ELECTRICALLY CONDUCTIVE PARTICLES AND RELATIVELY HIGH ELECTRICALLY RESISTIVE FILLER PARTICLES ALL BONDED TOGETHER IN PROXIMATE AND CURRENT-PASSING RELATION BY A HEAT RESISTANT ELASTOMER OF COPOLYMERIZED POLY SILOXANEOUS, AND A SUPPORTING MEMBER BONDED TO SAID ELEMENT AND COMPOSED OF A HEAT RESISTANT ELASTOMER OF COPOLYMERIZED POLY SILOXANES. 